Process for removing NOx from nitrosylsulphuric acid

ABSTRACT

Nitrosylsulphuric acid is mixed in a mixing reactor with sulphuric acid saturated with SO 2 . The acid mixture is led into a saturating reactor into the bottom region of which water and an SO 2 -containing gas are introduced. The gas partially serves as stripping gas inside the saturating reactor. SO 2 -saturated, No x -free sulphuric acid with 5 to 60 wt % H 2 SO 4  is removed from the bottom region of the saturating reactor and part thereof is mixed with nitrosylsulphuric acid in the mixing reactor.

This invention relates to a process of removing NO_(x) from nitrosyl hydrogensulfate by mixing nitrosyl hydrogensulfate in a mixing reactor with sulfuric acid, which is saturated with SO₂, where a sulfuric acid containing N₂O₃ is withdrawn from the mixing reactor, and a stripping gas is passed through the withdrawn sulfuric acid.

Such process is known from GB-A-0,348,866. For the expulsion of residual nitrogen oxides from the sulfuric acid flue gas or an inert gas is passed through the sulfuric acid, with the temperature lying in the range from 100 to 200° C.

The object underlying the invention is to remove NO_(x) from nitrosyl hydrogensulfate in a simple and inexpensive way. In accordance with the invention this is accomplished in the above-stated process in that the N₂O₃-containing sulfuric acid withdrawn from the mixing reactor is added to a saturation reactor, where in the lower portion of the saturation reactor an SO₂-containing gas is introduced at the same time, which at least in part flows upwards through the N₂O₃-containing sulfuric acid, that water is introduced into the saturation reactor, and from the saturation reactor a virtually NO_(x)-free sulfuric acid saturated with SO₂ is withdrawn, which has a H₂SO₄ concentration of 5 to 60 wt-%, and a partial stream of which is passed into the mixing reactor, where SO₂ is supplied to the mixing reactor in a stoichiometric surplus of at least 2 wt-% with reference to the NO_(x) content of nitrosyl hydrogensulfate.

In accordance with the invention, the term NO_(x) refers to a mixture of NO and NO₂. NO_(x) is present in sulfuric acid as dissolved nitrosyl hydrogensulfate. Nitrosyl hydrogensulfate is formed from NO, which is contained in SO₂-containing gas. The SO₂-containing gas originates for instance from a roasting, sulfur combustion, sulfate separation or metallurgical process. NO is oxidized at the oxidation catalyst of a sulfuric acid plant for up to 50% to form NO₂. The mixture of NO and NO₂ is reacted with sulfuric acid to form nitrosyl hydrogensulfate as follows:

NO+NO₂+2H₂SO₄→2HNOSO₄+H₂O

It is known that SO₂ reacts with nitrosyl hydrogensulfate by forming sulfuric acid and nitrogen oxides:

SO₂+2HNOSO₄+2H₂O→3H₂SO₄+2NO

In the production of sulfuric acid nitrosyl hydrogensulfate, usually with a NO_(x) content of more than 2.5 wt-%, is separated as a condensate. The NO_(x) content is present in the condensate as nitrosyl hydrogensulfate in addition to sulfuric acid.

In formal terms, nitrosyl hydrogensulfate can be reacted with SO₂ and water to form sulfuric acid and N₂. The redox reaction takes place between the dissolved SO₂ in the form of SO₃ ²⁻ with the N₂O₃ in the form of NO₂ ⁻, which was produced in the hydrolysis of nitrosyl hydrogensulfate. The reduction of the nitrogen oxides NO and NO₂ to nitrogen is effected in the hydrolysis of nitrosyl hydrogensulfate with dilute, SO₂-saturated sulfuric acid in that in the diluting solution the reducing agent is provided in the form of SO₃ ²⁻ in an overstoichiometric amount, so that the equilibrium is shifted towards the formation of N₂.

It is an advantage of the present invention that the amount sulfuric acid present in a condensate containing nitrosyl hydrogensulfate can be recirculated to a sulfuric acid production without any NO_(x). In accordance with the conventional processes the condensate is withdrawn from the process and must be subjected to a treatment or be disposed of. A particular advantage of the inventive process consists in that nitrogen is formed from the noxious substance NO_(x) and is discharged together with the exhaust gas.

Preferably, SO₂ is supplied to the mixing reactor in a stoichiometric surplus of at least 5 wt-% with reference to the NO_(x) content of nitrosyl hydrogensulfate. With this surplus of SO₂ a good result is achieved in the reduction of the NO_(x) content. SO₂ is not lost as a result of the reduction of NO_(x) to N₂, but is oxidized to form SO₄ ²⁻. Excess SO₂ can be supplied to a plant for the production of sulfuric acid, so that an extensive treatment is not necessary.

Advantageously, the sulfuric acid concentration after the addition of SO₂ is 5 to 30 wt-%. In this range of the sulfuric acid concentration particularly good results are achieved for the reduction of NO_(x) to N₂.

In accordance with an advantageous aspect of the invention gaseous SO₂ is brought into aqueous solution in a saturation reactor designed as packed column, and is combined with the NO_(x)-containing sulfuric acid as an aqueous solution. The SO₂ saturation is promoted by operating the saturator under an increased gas pressure.

In accordance with the invention, the NO_(x) content of a NO_(x)-containing sulfuric acid (nitrosyl hydrogensulfate) or a NO_(x)-containing sulfuric acid mixture is reduced. The process in accordance with the invention can advantageously not only be applied to NO_(x)-containing sulfuric acid, but with very good results also to mixtures containing NO_(x) and sulfuric acid, such as nitrating acid or sulfuric acid contaminated with other compounds.

Embodiments of the invention will be explained in detail with reference to the drawing and examples. The drawing represents a flow diagram of the process.

The most important parts of the process are the reactor (X) for the saturation of SO₂ and the mixing reactor (Y) for hydrolysis and redox reaction of nitrosyl hydrogensulfate with SO₂-containing dilute sulfuric acid. SO₂-containing roaster gas is introduced via line (1), and water is introduced via line (4) into the lower portion of the saturation reactor (X). Via line (3) nitrosyl hydrogensulfate or condensate containing said acid is supplied to the mixing reactor (Y). In principle, dilute sulfuric acid is circulated between the reactors (X) and (Y) via lines (5), (6), (6A), (7) and (8). Sulfuric acid formed is withdrawn via line (6B), and N₂ is discharged together with the exhaust gas via line (2).

Via line (5), sulfuric acid is withdrawn from the saturation reactor (X) and passed through a pump (9). Said sulfuric acid is saturated with SO₂, is virtually NO_(x)-free, and has a H₂SO₄ concentration of 5 to 60 wt-% and mostly not more than 35 wt-%. From the pump (9), the sulfuric acid is withdrawn via line (6). A partial stream of the acid is supplied to the mixing reactor (Y) via line (6A). The acid withdrawn from the reactor (Y) via line (7), which still contains N₂O₃, is supplied to the indirect cooler (W). Via line (10), the cooler is supplied with cooling water, which is withdrawn via line (11).

Cooled sulfuric acid is introduced into the saturation reactor (X) via line (8). The reactor (X) contains at least one packed bed. In the reactor (X) upwardly flowing SO₂-containing gas serves as stripping gas for removing residual nitrogen oxides from the sulfuric acid supplied via line (8).

EXAMPLE 1

In an arrangement as shown in the drawing the procedure is as follows:

Via line (3), 240 kg/h condensate with a nitrosyl hydrogen-sulfate content of 9.5% HNOSO₄ corresponding to 6.8 kg/h N₂O₃ are introduced into the mixing reactor (Y). The condensate comes from a sulfuric acid production. Via line (6A) an SO₂-saturated sulfuric acid with 20 wt-% H₂SO₄ is supplied to the reactor (Y), the condensate and SO₂-saturated sulfuric acid are mixed. The dissolved SO₂ reacts with nitrosyl hydrogensulfate to form sulfuric acid and nitrogen. The mixture is withdrawn via line (7), and behind the block cooler (W) still has a content of 465 mg N₂O₃/l, which corresponds to a content of 2.0 kg N₂O₃/h. In the saturation reactor (X) the circulating sulfuric acid is saturated with SO₂, where SO₂-containing roaster gas, which contains 0.28 g/h NO_(x), is supplied via line (1). The concentration of sulfuric acid is adjusted to 20 wt-% H₂SO₄ by means of a controlled addition of water through line (4). The roaster gas leaving the reactor (X) via line (2) has a reduced SO₂ content and is returned to the sulfuric acid production as a wet gas. The gas in line (2) contains 3.125 mg NO_(x)/Nm³ corresponding to 2.0 kg N₂O₃ per hour. In the sulfuric acid in lines (6B) and (6A) NO_(x) could no longer be detected. The denitrating conversion was 71.75%.

EXAMPLE 2

Example 2 is carried out like Example 1, but with the following differences:

SO₂-saturated sulfuric acid containing 16 wt-% H₂SO₄ is supplied to the mixing reactor (Y) via line (6A). Behind the block cooler (W) the sulfuric acid in line (8) contains 400 mg N₂O₃/l corresponding to 1.7 kg N₂O₃/h. After a controlled addition of water through line (4), a sulfuric acid with a concentration of 16 wt-% H₂SO₄ is produced in the saturation reactor (X) and withdrawn via line (5). This sulfuric acid is NO_(x)-free. The denitrating conversion was 80.23%.

EXAMPLE 3

The procedure is as in Examples 1 and 2, but with the following differences:

Via line (3) 190 kg/h condensate are introduced into the mixing reactor (Y). The condensate has a nitrosyl hydrogensulfate content of 7.5 wt-% HNOSO₄ corresponding to 4.3 kg N₂O₃ per hour. The condensate is mixed in the mixing reactor (Y) with a 33%, SO₂-saturated sulfuric acid from line (6A). Behind the block cooler (W), the sulfuric acid in line (8) contains 350 mg N₂O₃/l corresponding to 1.5 kg N₂O₃/h. In the saturation reactor (X) a sulfuric acid with a concentration of 33 wt-% H₂SO₄ is adjusted through a controlled addition of water and withdrawn through line (5). The acid is NO_(x)-free. The SO₂ gas withdrawn via line (2) contains 4700 mg NO_(x)/Nm³ corresponding to 3.0 kg N₂O₃/h. The denitrating conversion is 34.5%.

The comparison of Examples 1, 2 and 3 clearly illustrates that there is a dependence between the NO_(x) conversion and the concentration of the circulating sulfuric acid. The higher the concentration of the circulating sulfuric acid, the lower the denitrating conversion.

The advantage of a rather high denitrating conversion consists in the fact that the circulating amount of NO_(x) is lower. NO_(x) which is not discharged from the denitrating plant gets back to the main gas stream of the sulfuric acid plant, where nitrosyl hydrogensulfate is formed again. A lower denitrating conversion requires a larger denitrating plant. 

What is claimed is:
 1. A process of removing NO_(x) from nitrosyl hydrogensulfate by mixing nitrosyl hydrogensulfate in a mixing reactor with sulfuric acid, which is saturated with SO₂, where a N₂O₃-containing sulfuric acid is withdrawn from the mixing reactor, and a stripping gas is passed through the withdrawn sulfuric acid, wherein the N₂O₃-containing sulfuric acid withdrawn from the mixing reactor is added to a saturation reactor, where in the lower portion of the saturation reactor a SO₂-containing gas is introduced at the same time, which at least partly flows upwards through the N₂O₃-containing sulfuric acid, that water is introduced into the saturation reactor, and from the saturation reactor an SO₂-saturated, virtually NO_(x)-free sulfuric acid with a H₂SO₄ concentration of 5 to 60 wt-% is withdrawn, a partial stream of which is introduced into the mixing reactor, where SO₂ in a stoichiometric surplus of at least 2 wt-% with reference to the NO_(x) content of nitrosyl hydrogensulfate is supplied to the mixing reactor.
 2. The process as claimed in claim 1, wherein the sulfuric acid withdrawn from the mixing reactor is cooled before being introduced into the saturation reactor.
 3. The process as claimed in claim 2, wherein the sulfuric acid withdrawn from the mixing reactor is indirectly cooled with cooling water, where heated cooling water is withdrawn from the indirect cooling system.
 4. The process as claimed in claim 1 wherein SO₂ in a stoichiometric surplus of at least 5 wt-% with reference to the NO_(x) content of nitrosyl hydrogensulfate is supplied to the mixing reactor through the partial stream of SO₂-saturated sulfuric acid.
 5. The process as claimed in claim 1 wherein SO₂-saturated sulfuric acid with a H₂SO₄ content of 5 to 35 wt-% is withdrawn from the saturation reactor.
 6. The process as claimed in claim 1, wherein the nitrosyl hydrogensulfate is obtained from a plant for producing sulfuric acid from SO₂-containing roaster gas, and SO₂-containing roaster gas is introduced into the lower portion of the saturation reactor. 